Semipermeable membranes play an important part in industrial processing technology and other commercial and consumer applications. Examples of their applications include, among others, biosensors, transport membranes, drug delivery systems, water purification devices, supported catalysts, including supported enzyme catalysts, and selective separation systems for aqueous and organic liquids carrying dissolved or suspended components.
Generally, semipermeable membranes operate as separation devices by allowing certain components of a liquid solution or dispersion of solvent and one or more solutes to permeate through the membrane while retaining other components in the solution or dispersion. The components that permeate or are transmitted through the membrane are usually termed permeate. These components may include the solution or dispersion solvent alone or in combination with one or more of the solution or dispersion solutes. The components retained by the membrane are usually termed retentate. These components may include either or both of the solution or dispersion solvent and one or more of the solution or dispersion solutes. Either or both of the permeate and retentate may provide desired product.
The industry has, for convenience, categorized these semipermeable membranes as microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes. These categories do not have rigid definitions. Most definitions available in the industry arrange the membranes according to properties and function. For example, the microfiltration and ultrafiltration membranes are often defined by their pore sizes. Typically, these membranes contain recognizable pores of sizes from 0.1 to 10 microns and 1 nm to 0.1 micron respectively. Nanofiltration (NF) and reverse osmosis (RO) membranes, in contrast, are most often regarded as not containing recognizable pores. Instead, NF and RO membranes are believed to transmit liquid permeate through void spaces in the molecular arrangement of the material making the membrane barrier layer. NF membranes typically are used, for example, to fractionate monovalent ions from divalent ions or to fractionate small organic compounds from other small organic compounds (monosaccharides from disaccharides, for example) or salts from organic compounds. RO membranes generally retain all components other than the permeating liquids such as water, with certain exceptions such as weakly ionizing HF, which tends to permeate with water through RO membranes. Under certain circumstances, the RO membranes can also be used to separate and/or fractionate small organic molecules.
RO membranes are often found in industrial applications calling for concentration of mixtures of inorganic salts, or concentration of mixtures of small, very similar organic molecules. RO membranes are used foremost for desalination either of municipal or well water or of seawater. These membranes are also typically used in recovery operations such as mining, spent liquor recovery from industrial processing and general industrial applications. The RO membranes function by retaining the solution solute, such as dissolved salts or molecules, and allowing the solution solvent, such as water, to permeate through the membrane. Commercial RO systems typically retain greater than 99% of most ions dissolved in a solvent such as water.
In contrast, NF membranes are often found in industrial applications calling for separation of one small compound from another. For example, NF membranes are used foremost for separation of alkaline salts from alkaline earth salts such as separation of mixtures of sodium and magnesium chlorides. Some NF membranes function by retaining the double charged ions while allowing the singly charged ions (with their corresponding anions) to permeate with the solvent.
RO and NF membranes are typically characterized by two parameters: permeate flux and retention ability. The flux parameter indicates the rate of permeate flow per unit area of membrane. The retention ability indicates the ability of the membrane to retain a percentage of a certain component dissolved in the solvent while transmitting the remainder of that component with the solvent. The retention ability is usually determined according to a standard retention condition.
RO and NF membranes are typically operated with an appropriate pressure gradient in order to perform the desired separations. When functioning to separate, the filtration process using a RO or NF membrane overcomes the osmotic pressure resulting from the differential concentration of salts on the opposing sides of the membrane. Under an unpressurized situation osmotic pressure would cause solvent on the side with the lower salt concentration to permeate to the side having the higher salt concentration. Hence, pressure must be applied to the solution being separated in order to overcome this osmotic pressure, and to cause a reasonable flux of solvent permeate. RO membranes typically exhibit satisfactory flow rates, or fluxes, at reasonable pressures. Currently, typical commercial RO systems have fluxes on the order of 15 to 50 lmh (liters per m2 per hour) at about 7 to 30 atmospheres pressure, depending on the application. Home RO systems typically run at lower pressures (1-6 atmospheres depending on line pressure) and lower fluxes (5 to 35 lmh). Seawater desalination typically runs at higher pressures (40 atm to 80 atm) and fluxes in the range of 10 lmh to 30 lmh. RO membranes also have advantageous salt retention characteristics. For example, to purify seawater, an RO membrane will typically have a salt retention value of at least 98.5 percent and preferably 99 percent or more, such that the total ion retention ability for commercial RO treatment of seawater typically will be in excess of 99.5%.
The majority of semipermeable membranes functioning as RO and NF membranes are cellulose acetate and polycarboxamide (hereinafter polyamide) membranes as well as sulfonated polysulfone and other membranes for NF alone. Polyamide membranes often are constructed as composite membranes having the thin polyamide film formed as a coating or layer on top of a supporting polysulfone microporous membrane. Typically, the RO or NF membrane is formed by interfacial polymerization or by phase inversion deposition. For example, U.S. Pat. No. 3,744,642 to Scala discloses an interfacial membrane process for preparation of an RO or NF membrane. Additional U.S. patents disclosing polyamide and polysulfonamide membranes include U.S. Pat. Nos. 4,277,344; 4,761,234; 4,765,897; 4,950,404; 4,983,291; 5,658,460; 5,627,217; and 5,693,227.
Several characteristics are described in these and other U.S. patents pertaining to semipermeable membranes as factors for advantageous operation of RO and NF membranes. These characteristics include high durability, resistance to compression, resistance to degradation by extremes of pH or temperature, resistance to microbial attack, and stability toward potentially corrosive or oxidative constituents in feed water such as chlorine. Although the polyamide membranes typified by U.S. Pat. No. 4,277,344 are widely used, especially in desalination operations to purify water, these membranes are susceptible to corrosive attack, as well as low pH and temperature degradation. Furthermore, microbial fouling of the membrane can cause loss of flux and/or retention characteristics. Nevertheless, current polyamide membranes substantially reach the goals of minimal thickness and substantial freedom from flaws or imperfections, allowing for widespread commercial use.
These two goals of minimal thickness and freedom from flaws, however, are not altogether compatible. As the thickness of the polymeric film or membrane decreases, the probability of defect holes or void spaces in the film structure increases significantly. The defect holes or void spaces result in significant loss of solute retention.
Polysulfonamide membranes provide several possible advantages over polyamide membranes. Although polysulfonamide membranes have been reported, they have no appreciable commercial application. Generally they have poor flux rates and low solute retention capabilities. For example, B. J. Trushinski, J. M. Dickson, R. F. Childs, and B. E. McCarry have described investigations of polysulfonamide membranes and their modifications in the course of attempts to achieve higher flux and better retention abilities. Trushunski, Dickson, Childs, and McCarry report these attempts in the Journal of Membrane Science 143, 181 (1998); Journal of Applied Polymer Science, 48, 187 (1993); Journal of Applied Polymer Science, 54, 1233 (1994); and Journal of Applied Polymer Science, 64, 2381 (1997). Trushunski, Dickson, Childs, and McCarry however, have been unable to achieve the functional properties of the polyamide membranes using polysulfonamides. Those functional properties are believed to enable at least in part the achievement of the typical performance thresholds qualifying a membrane for practical use.
Therefore there is a need for polysulfonamide membranes that display flux and retention capabilities like those of the polyamide membranes. In addition, there is a need to develop semipermeable membranes such as RO and NF membranes that are stable to strong acid conditions and/or stable to oxidative conditions. There is a further need to develop semipermeable membranes that will be useful in heavy, corrosive industrial applications including mineral mining, industrial desalination, industrial waste purification, industrial and residential recycling and solute recovery.